Patrick Corbin is off to a good start in 2018. However, a few red flags have emerged that should concern the D-backs.

The Arizona Diamondbacks currently have very little to complain about. With a 25-18 record through May 17th, the Diamondbacks have one of the best records in the National League. This is remarkable considering how anemic the offense has been so far this season.

Paul Goldschmidt’s struggles (a .208 batting average with only four home runs and 12 RBIs) and being without Steven Souza Jr. and Jake Lamb have predictably harmed an offense expected to be significantly better. The Diamondbacks rank last in batting average (.220), 28th in OBP (.300), and 25th in runs scored (170), numbers that would indicate that the Diamondbacks should not be doing as well as they are.

What is currently propelling the team to success is their pitching, particularly their starters. Despite being without Taijuan Walker, the Diamondbacks’ pitching staff has been great: 2nd in ERA (3.31), 4th in quality starts (25), 3rd in BAA (.222), and 3rd in WHIP (1.16). Part of this success can be credited to Patrick Corbin, who has arguably been the team’s best starter thus far. Consider his overall performance statistics below:

In every major statistical pitching category, Corbin ranks in the top five. Most impressive has been his ERA and WHIP, both career-highs.

It helps that Corbin has been a beneficiary of hitters increasingly striking out this year. In 2017, MLB hitters struck out at a previously record-high 21.6% of plate appearances. So far in 2018, the strikeout rate has increased to 22.3% of plate appearances. In 2017, Corbin averaged 8.4 strikeouts per nine innings. In 2018, Corbin has struck out three more batters per nine innings, averaging 11.8 strikeouts per nine innings thus far.

Although a slight uptick in strikeouts can be expected, it still does not explain Corbin’s sharp increase in strikeouts. Corbin’s contact percentage against (the percentage of contact made by hitters when swinging at all pitches) has dropped from 76.2% to 66.2%, but it also does not necessarily explain by itself why Corbin is striking out more hitters.

In order to see what has Corbin fanning more batters, his pitch selection is the first thing that must be looked at. From the start of last year, Corbin made two significant changes to his repertoire: he has been throwing more sliders, especially late in the count, and has also been phasing out his four-seam fastball in favor of his sinker.The chart below highlights this shift in pitch usage:

In 2016, Corbin’s four-seam fastball percentage straddled the 40% mark. However, for the past two years, Corbin has reduced his fastball usage from around 30% in 2017 to 19% in 2018. The 2018 season also brought about one more significant change in Corbin: his change-up has been nearly abandoned in favor of more sinkers and curveballs. As a result, he effectively substituted his change-up out of his routine.

Also noticeable is that Corbin, who was known as a pitcher that pounded the strike zone, is increasingly throwing more pitches out of the strike zone. Over the course of his career and especially in 2018, his zone percentage (the number of pitches he throws in the strike zone) has decreased, suggesting that Corbin is learning how to get hitters to chase pitches out of the zone.

Through the first seven weeks of the 2018 season, Corbin’s slider has undoubtedly been the key to his success. His command of it, along with his increased usage of the pitch, has led many hitters to chase at more of his pitches, resulting in more high-strikeout starts.

However, not everything is looking great for Corbin. As April turned into May, Corbin has shown signs that may indicate he is due for a sharp decline shortly. One of these signs is his drastic decrease in velocity.

The chart shows a sample of 145 starting pitchers this year with the yellow dot representing Corbin. On average, a starting pitcher’s velocity from April to May has improved by 0.2 miles an hour. Corbin’s, on the other hand, has declined three miles an hour.

His past two starts, in particular, have been worrisome for his velocity. Zach Buchanan wrote in The Athletic that Corbin insists he is okay. Despite his fastball velocity decreasing from 89-to-90 mph to 87-88 mph, it is easy to see why Corbin said he feels fine. With a 1.64 ERA, 1.09 WHIP, and 12 strikeouts in 11 innings over his past two starts, Corbin has not shown noticeable decline.

But clearly, something is not right with Corbin. A 28-year-old pitcher who is only averaging 94.1 pitches per start is throwing fastballs as though he is getting burned out. Take a look at his first six starts compared to his seventh and eighth:

In this chart, the red line shows his maximum velocity on his fastball. The yellow line averages Corbin’s 10 fastest fastballs each start and the blue line tracks the average velocity of all fastballs he throws. The seventh and eighth starts are haunting because Corbin has not even touched 92 mph once on a fastball. It is true that occasionally a pitcher would dial back their fastball intentionally for one reason or another. However, it should still be expected that a pitcher would be able to show at least once during a start that he can throw a powerful fastball. Corbin has not shown in his past two starts that he is capable of bringing even average zip on his fastball.

His decreased usage of his fastball may be a result of Corbin realizing it is not his best pitch. Lately, he has been dealing with another problem: the command of his slider has worsened from the start of the season. Seeing as Corbin has identified his slider as the pitch that has been the most effective so far, this is more worrisome than his fastball’s velocity.

This was evident in his start against the Dodgers, which was his eighth start in the 2018 season. Throughout his start, Corbin’s command on his slider was shaky at best. He threw 14 pitches in the dirt, with his previous high being only seven. His slider contributed to his allowance of four walks in five innings along with only 50 of his 96 pitches being strikes. Even Corbin admitted that he had a rough start.

Is Corbin’s shaky command something to worry about as the season progresses? His poor performance (6 IP, 4 ER) against the Brewers following his shaky start against the Dodgers serves as another point of evidence that something is not right with him. If Corbin continues to struggle with his slider, things may get ugly for the Diamondbacks ace.

(All stats and information came from ESPN.com, FanGraphs.com, and Baseball-Reference.com, unless otherwise noted.)